The present invention relates to a switch unit, particularly a reed switch unit to be utilized for detecting a liquid level in a reservoir tank in a master cylinder.
A brake system employed in a driving automobile is so constructed that a brake liquid circulates in a hydraulic circuit which is formed between the master cylinder adapted to be connected to a brake pedal or the like and a wheel cylinder adapted to be connected to a wheel brake. To the master cylinder is fitted a reservoir tank in which the brake liquid is stored to be supplied to the master cylinder.
In order to assure an accurate operation of the brake system, it is necessary that more than a determined amount of the brake liquid is always stored in the reservoir tank. For this reason, there is provided in the reservoir tank a liquid level detecting sensor for detecting the liquid level of the brake liquid stored therein, in order to issue a warning to a driver, etc. when the brake liquid has decreased below a prescribed amount.
The liquid level detecting sensor consists of a float which moves up and down with a vertical movement of the liquid level of the brake liquid and has a permanent magnet, and a reed switch unit having a reed switch for detecting the vertical movement of the float. The reed switch is an electric component of such a structure that its contacts are adapted to be closed when the permanent magnet in the float approaches the reed switch.
A conventional reed switch unit is shown in FIGS. 1 and 2.
The reed switch unit 10 is composed of a reed switch 11 and a unit body 12 which is formed of a synthetic resin and on which the reed switch unit 11 is mounted.
The reed switch 11 is so constructed that a contact part 14 of which contacts are adapted to be closed in association with an approach of the permanent magnet is sealed inside a glass tube 16. In the glass tube 16 is sealed an inert gas (for example, nitrogen gas) in order to prevent oxidation, frosting, etc.
The glass tube 16 is provided at its opposite ends with lead portions 18 which are adapted to be connected to the contact part 14.
The unit body 12 formed of a synthetic resin comprises a coupler part 12a which can be connected to another electric circuit on the right hand in the drawing, and a switch receiving part 12b on which the reed switch 11 is adapted to be mounted on the left hand in the drawing.
The switch receiving part 12b is provided with metal terminal members 20 to be connected with the respective lead portions 18, 18 of the reed switch 11. Each of the terminal members 20 is formed of a plate-like member provided with a slit 22 which can grasp the lead portion 18 and projecting upward from the switch receiving part 12b. The terminal members 20 are respectively connected to coupler terminals 23 which are shown by a dotted line in the coupler part 12a.
The reed switch 11 is fitted to the unit body 12 and fixed by soldering after both the lead portions 18, 18 have been grasped in the slits 22 of the terminal members 20. Because the lead portions 18 and the terminal members 20 are fixed by soldering, both physical fixations and electrical connections of the lead portions 18 can be assured.
In order to help positioning of the glass tube 16 when the reed switch 11 is fixed to the terminal members 20, the unit body 12 has, at a position where the glass tube 16 is to be located, ribs 24 which are integrally formed with the unit body 12 and upwardly project from the switch receiving part 12b so as to surround side faces of the glass tube 16.
By providing the ribs 24, when the reed switch 11 is mounted, the glass tube 16 is first placed between the ribs 24, and then fixed by soldering after the lead portions 18 are grasped in the slits 22 of the terminal members 20. Therefore, the reed switch 11 is securely fixed at a determined position (For example, see a Japanese Patent Unexamined Publication No. Hei.7-220817).
As described above, it has been a conventional manner that the lead portions and the terminal members are fixed by soldering, when the reed switch is mounted on the unit body.
However, there is such a problem that in case the soldering should be conducted at two points at the same time when the lead portions and the terminal members are connected, it is difficult to solder at the two points at the same time because a stress occurs in a direction of pulling the glass tube. There is a further problem that the soldering takes a long heat introducing time into components to be connected, and a line tact in the process will be extended. Moreover, after the soldering has been carried out, the solder will be inevitably splashed, and in case that the solder has entered between the glass tube and the unit body, there will be a fear that the glass tube may be broken. This will lead to a decrease in productivity because scattering of the solder must be detected and so on.
Further, there is still another problem that since the soldering takes a long heat introduing time, the heat will be easily conducted to the glass tube or the connected parts which are in contact with the lead portions, and the inert gas in the glass tube will escape.
Further, a quality control is difficult with the soldering, because it results in a faulty joint in case where the heat introducing time is too short or the faces to be connected are not clean.
In addition to this, a conventional reed switch unit for the reservoir tank has been also known as disclosed, for example, in Japanese Patent Unexamined Publication No. Hei.4-325362.
The conventional reed switch unit for a reservoir tank to be mounted on the reservoir tank is used for storage of operating liquid or the like in a master cylinder. The reed switch unit is contained in a switch housing formed in the reservoir tank in isolation from an interior thereof and comprises a reed switch, a first and a second terminal members and a coupler part.
The first and a second terminal members respectively have a first and a second connecting terminals respectively connected to a first and a second reed parts of the reed switch, and a first and a second coupler terminals at their respective opposite ends. The coupler part is formed of a synthetic resin and connected by molding to an intermediate portion between the first and the second terminal members to receive the first and the second coupler terminals.
In the conventional reed switch unit for the reservoir tank, because the first and the second terminal members to which the coupler part of the synthetic resin is connected by molding are individually manufactured, it has been difficult to accurately set the first and the second terminal members at respective fixed positions in metal molds on occasion of molding the coupler part. This has been a drawback when cut-down of the manufacturing cost is intended.